The newest recording by Mstislav Rostropovich is the most puzzling of his multi-decade career. Heard on a new ECM-label disc titled Amicta Sole by Russian composer Alexander Knaifel, Rostropovich seems to be noodling around on his cello, practicing those difficult-to-play pitches in his upper range. The tone quality is pale, the same half-dozen notes are repeated, and the rhythm isn't exactly snappy.

Yet there's nothing accidental or casual about it. The title of the piece is Psalm 51, but the words - a heartfelt plea for mercy - are never heard. They're thought, by the cellist.

Each note he plays, according to the disc notes, is assigned to a word in a line of the text. It's a hymn without words. It's the sound of a cellist praying. At least, one assumes so. Will we ever know if Rostropovich is really thinking about dinner?

The radical fringe of serious music isn't on Manhattan's Lower East Side or at George Crumb's house in suburban Philadelphia. It's among the mystical and the devout of Eastern Europe and Russia, collectively called "holy minimalists" - and they're championed by some of the world's best-known performers. Spare in the extreme, might the music be too much an afterthought of the composers' inner experience? Or do we just need a few decades to figure it out?

Holy minimalism first reached America via Henryk Gorecki's Symphony No. 3, which was written in 1976, but became a surprise best-seller on CD in 1992. Now, that piece sounds like Richard Strauss in comparison to the musical asceticism that has followed.

Composers since then such as Giya Kancheli, Arvo Pärt and now Knaifel were commercially validated by the Benedictine Monks of Santo Domingo de Silos - who weren't even good singers but showed that the world hungers for an authentic spiritual experience in music. The result is that the holy minimalists have been writing pieces so ethereal that their gentle spells could be broken by a mere cough. Rather than being a statement of faith, holy minimalism echoes faith with a few notes played slowly and contemplatively, seeming to float in midair.

That's why this music isn't often on concert programs, but is best heard in the pristine quiet of compact disc - through which Pärt, now turning 70, has become one of the world's beloved composers. Had he existed in predigital technology, he might be all but unknown.

American-made religious music is more insistent about being heard and understood. For example, the foremost sacred composer, Morten Lauridsen, writes music that's straightforward, conventionally beautiful and seeks to comfort. Social and political reasons for the differences are obvious. Because religious lobbies have become more politically active in recent decades, doctrine is well-defined and more public than ever. So in an art form as elusive as music, ambiguity of expression is likely to lead to misunderstanding.

That's what Scottish composer James MacMillan discovered three years ago in Philadelphia when his work Quickening, written in praise of the Blessed Virgin Mary with choruses of unborn children, was assumed to be an antiabortion tract. Emphatically not so, he said, explaining that his idea of spiritual expression is "messy and disturbing." He's a maverick who asks questions; present-day American sacred music gives answers.

Contrast all of this with the Eastern Europeans. Composers emerging from the former Soviet Union were born (Knaifel in 1943) under a regime in which religious practice was officially banned. Making a joyful noise in the wrong place could land you in Siberia. Therefore, their music comes out of a history of quieter, more private dialogues with their God. They imply more by saying less, leaving large spaces for widely varying listener reaction. At its best, this music emerges as a meticulously chosen series of notes and a product of true vision.

That's the case with Knaifel's choral work Amicta Sole (Clothed With the Sun), which shares the ECM disc with Rostropovich. The vocal writing sounds like female voices calling to you from a long tunnel of white light. Most radical, the music seems directionless. Music from nirvana, after all, has achieved the ultimate arrival. This approach, however, is tricky: Knaifel's Psalm 51 (the Rostropovich piece) seems like barely organized sound.

What probably gives Knaifel the courage to be this singular arises from how composition works in his life - a process that's about maintaining an inner equilibrium and is dictated by a profound, difficult-to-define experience.

This singularity spills over into nonsacred works. Pärt's Lamentate, a piano concerto of sorts that comes out on ECM on Aug. 30, has long passages of the piano sounding one note at a time while the orchestra plays a quiet, steady heartbeat. That's a long way from Brahms. Kancheli's violin/piano sonata, Time... and again, has the two instruments dialoguing vaguely, seemingly from different dimensions. Try to figure out the long-term organization - usually the key to comprehension and the hallmark of great composers of centuries past. I can't.

However, Beethoven's divinely inspired, late-period Missa Solemnis was initially considered obscure and nearly unperformable before emerging as one of the great masterpieces of Western civilization. That's why holy minimalists demand provisional evaluations. They know something I don't.
Contact music critic David Patrick Stearns at 215-854-4907 or dstearns@phillynews.com. Read his recent work at http://go.philly.com/davidpatrick stearns.

"Only two things are infinite, the universe and human stupidity, and I'm not sure about the former."